(i) Field of the Invention
The present invention relates to a pre-plasticizing injection molding machine with a plasticizing component provided separately from an injection component and, more particularly, relates to the provision of additional melt channels and redesigned shooting pot head for minimizing material retention and stagnation in the shooting pot.
(ii) Description of the Related Art
A conventional two stage or pre-plasticizing injection molding machine comprises, in general, a plasticizing component having a screw in a heated barrel and an injection component having an injection plunger. Polymer resin plasticized in the plasticizing component is transferred to the injection component's chamber, or shooting pot, by way of a feed channel and is injected into the mold cavity through a nozzle from the injection plunger. Conventionally, a distributor or ball check valve in the feed channel prevents resin in the shooting pot from flowing backward to the plasticizing component when the injection plunger injects the resin.
In that the pre-plasticizing injection molding machine is equipped with an independent plasticizing component, it has more plasticizing capability than an in-line or reciprocating screw type injection molding machine. However, the ball check valve often causes resin retention and stagnation in the shooting pot, and such resin is prone to thermal degradation. For polyethylene teraphthalate (PET) resins, increased residence time in the shooting pot leads to higher levels of acid aldehyde (AA), an unacceptable by-product of the manufacturing process.
It is also necessary to completely purge any remaining resin in the shooting pot prior to changing resin material and/or color. If a small residual portion of resin remains in the shooting pot, purging must be repeatedly carried out until the residue is removed. This leads to more scrap waste and to delays in the manufacturing process.
Several techniques have been developed in an effort to expedite changes in resin material and/or color and to minimize down time and scrap waste.
U.S. Pat. No. 5,380,186 issued Jan. 10, 1995 discloses a flow regulating assembly for an in-line or reciprocating screw plastic injection molding machine in which plastic flow from a central chamber in an injection piston is directed towards the perimeter of the injection piston adjacent the wall of the accumulator cavity, thereby scouring and displacing stagnant plastic material otherwise collecting in corners.
U.S. Pat. No. 5,380,187 issued Jan. 10, 1995 discloses a nozzle adapter for a pre-plasticizing injection molding machine comprising an injection by-pass from the perimeter of the injection chamber to the injector nozzle for discharging resin from the previous injection to the injector nozzle. This design eliminates a key advantage of a pre-plasticizing machine, namely the ability to continue plasticizing melt during injection, because the screw must be held forward during filling of the shooting pot, injecting and holding (clamping) to prevent backflow of resin. Furthermore, the infeed channels are smaller in diameter than the outflow channel to the injection nozzle, resulting in slower filling times. Both factors impose a significant cycle time penalty.
U.S. Pat. No. 5,616,350 issued Apr. 1, 1997 discloses a dual flow divider for use on an extruder for balancing feed to two or more extrusion dies and for streamlining resin flow to avoid hot spots and stagnation.
It is an object of the present invention to provide a pre-plasticizing injection molding machine that minimizes resin retention and stagnation in the shooting pot.